1. Field of the Invention
This invention relates to a system for removing particles from exhaust produced by internal combustion engines, such as diesel engines.
2. Description of the Prior Art
Exhaust from diesel engines has a relatively high content of polluting particles composed of carbon, unburned fuel, and partially burned fuel. Filters disposed in engine exhaust systems are conventionally used to remove the particles from the exhaust. As the amount of particles trapped by the filter increases, an engine exhaust back pressure rises and the efficiency of the filter decreases. When the degree of clogging of the filter is unacceptable, the trapped particles should be removed to unclog and rejuvenate the filter.
Most diesel engines have unthrottled air intakes wholly independent of the fuel supply so that the air/fuel mixture is always much leaner than stoichiometric. However, some recently developed engines employ intake valves capable of throttling the air flow and thus increasing the temperature of the engine exhaust.
Japanese patent publication No. 58-51235 discloses an exhaust particle removing system of this sort including a filter and an air flow throttle valve. In this system, when the exhaust filter is to be rejuvenated, the throttle valve reduces the rate of intake air flow, increasing the temperature of engine exhaust and thereby burning off the particles deposited on the filter. This system also includes a device for determining whether or not engine operating conditions derived from the engine speed and the engine load are suited for throttling of the intake air flow. When the engine operating conditions are determined to be unsuitable for unclogging of the filter, the air flow rate is not throttled even if the filter must be rejuvenated. Furthermore, during unclogging of the filter, the position of the throttle valve is adjusted so that the presure at a point within the engine air intake system downstream of the throttle valve will remain at a predetermined constant level corresponding to the lower limit of a range where fuel ignites reliably.
The exhaust particle removing system disclosed in the above-mentioned Japanese document has some drawbacks. First, at high engine loads, the reduction of the air flow rate needed to unclog the filter is performed to excessive degrees so that the engine fuel economy deteriorates and smoke emission increases. Second, it is difficult to finely adjust the degree of throttling of the intake air flow. In addition, the exhaust particle removing system is not designed for use with an exhaust gas recirculation (EGR) system used to reduce engine NOx (nitrogen oxides) emissions.